1920's era Athol Vise

I've received a few inquiries about my Athol vise, so I've made it a separate entry with additional pictures. What makes this vise unusual is the dog clutch in the handle that allows the handle to be repositioned by pulling straight out on the handle and rotating.

These teeth are rounded just enough to allow the clutch to slip under load. I removed the spring and tightened down on the nut to prevent it from disengaging.

A 14/20 screw holds the handle on the shaft

These teeth are also worn.

The spring went in here.  I removed it and made a sleeve to go in it's place to prevent the clutch from disengaging.

3 Years of Solar Hot Water Data

Here is some of the most comprehensive data on solar hot water performance you will find anywhere:  3 years of daily records for my 80 gallon Reynolds solar hot water system.  It has performed remarkably well, year round.  The number of days it did not run at all is surprisingly small and consistent:  53 days in 2009, 49 days in 2010 and 48 days in 2011. The number of days it reached 100% of capacity is 120 days in 2009, 181 days in 2010, and 190 days in 2011. The number of hours it ran is a little less consistent:  1990 hours in 2009,  1888 hours in 2010 and 1651 hours in 2011.  While it may seem odd that 2011 has both the fewest hours and the most 100% days, it is because the system needed to run less, as the storage tank was frequently still hot from the day before.  Unfortunately, I don't have an hour meter on the electric heating elements, or a water meter to measure actual use.  I estimate that we use between 30 and 40 gallons of hot water each day.  Since the system uses only 175 watts when running, it is using less than $50/year in electricity, while saving over $400.  The system has paid off the initial investment, and is now giving me a return in excess of 30%.  

         This is one form of renewable energy that is truly competitive with conventional sources.  Here are the reasons why:

            1:  The systems are uncomplicated.  Two panels with antifreeze circulating in them, a heat exchanger, a pair of small pumps, and a standard 80 electric hot water heater tank.

            2: They are efficient.  Hot water panels are approximately 40% -60% efficient vs.  14% for solar electric panels.

             3: The systems are reliable.  My panels, heat exchanger and pumps are all 28 years old, with plenty of life left in them.

            4:  Unlike solar or wind electric, the energy is easily stored for later use.  It usually takes 3 overcast days for my system to go cold.  In 2011, there were only 23 days where the system was below 90 degrees. Even 90 degrees provides a significant savings, as my electric heater does not have to heat 60 degree well water. Heating 90 degree water reduces my electric consumption by 30% vs. heating 60 degree water.

In 2009, the system made 63% of our hot water

It 2010, it made 77%

In 2011, a record 80%

Finally, some suggestions for those considering installing a system.  If your latitude is in the 40's, angle your panels to at least 45 degrees.  Why?  Because if you have them at a lower angle, they will overheat in the summer, and under-perform in the winter.  Ideally, the system is running through most of a hot summer day, which prevents the panels from overheating.  When the antifreeze is not circulating, the panels can easily reach 220 Fahrenheit, and the antifreeze will begin to break down and become acidic. Not good for the pipes.  An angle of 45 degrees or higher will reduce the amount of direct sun exposure in the summer, while still providing plenty of hot water.

The 45 degree angle helps in the winter, when the sun is low in the sky.  The panels will capture a good amount of winter sun.  Even in my worst winter, 2009, the system was making 40% of my hot water.  Another advantage is that snow readily slides off the panels.  I rarely lost more than a day due to snow coverage.  If it was a sunny day, I would often turn on the pumps manually to defrost the panels. It wasn't long before the snow slid off and the panels were capturing more energy than the defrost mode used.  This method worked with as much as 8 inches of snow on the panels.

I believe that for most people, evacuated tube collectors are unnecessary and simple flat panels are a better choice. Here my explanation why.

This is a picture of the panels on the day after a snowstorm that deposited about 12 cm of snow on the panels. It is 10:45 AM, and the outside temperature is -12c.  A small area of the panels was exposed by the wind, and that was enough to begin to warm the panels the panels to 38c. The pumps started up, and soon the entire system was at 38c.  That rapidly melted the remainder of the snow.  By 11:40, the panels were 80% clear, and the system temperature was now 57c.
 
These pictures also illustrate why I believe that solar panels should be installed at a minimum angle of 45 degrees in northern climates.  The snow readily slides off, and the panels are well positioned to capture the winter sun.  They produce more than enough hot water in the summer, even though the sun is then higher than 45 degrees.

In only 3 hours, the system had heated  300 liters of water from 23c to 43c, while the outside temperature never exceeded -10c, and the winds averaged 22kph. 

Unfortunately I have no comparable data on the performance of evacuated tube collectors under these conditions, but this performance is very good, especially considering these panels are now 29 years old.  It is unlikely that I will ever have to replace these panels, for given their current condition, I estimate their working life to be approximately 50 years.  But if I did replace them, I would stay with flat panels

Upper right. Discharge temperature from 300 liter storage tank.


Lower right. Return temperature of antifreeze in panel loop.  This temperature is after the heat exchanger. The water entering the exchanger from the panels is about 58c. 

Repairing a RapMan Controller

Has the extruder stepper driver circuit failed in your RapMan or BFB 3000 controller board?

Here is how I fixed mine in about 20 minutes using only a single piece of wire!

The BFB board Version 3.3 Part #30003 is used in both the RapMan 3.1 and the BFB 3000. These boards have 3 Extruder outputs.  That means your RapMan 3.1 has a built-in spare!  Or TWO spares if you are running a single head.  The trick is how to access those spares?  While in theory it could be done in the firmware, I don't have the ability.  So, I went for a hardware based solution. I began by tracing the circuits until I was able to identify the critical wires.   It turns out that BFB made things exceptionally easy by sending all 3 extruders the same signals in parallel, and only making the "Enable"  signals separate.  That means that only one jumper is needed from the Enable #1 trace to the Enable #3 trace.  The only other thing that is needed is to cut the traces leading TO stepper driver #1, and FROM the CPU to stepper driver #3.  After that, just move the wires on the 25 pin connector from Stepper #1 to Stepper #3. 

The driver chip is an Allegro A3979, and is available from various distributors.  Replacing them is challenging, for the bottom of the chip is soldered directly to the board, which acts as a heat sink.  This means that you can't simply heat the pins and remove the chip.  I removed mine by heating the opposite side of the board with a large soldering iron after cutting the pins free and removing them.  Unfortunately, I damaged the board in the process.  That is when I decided to abandon the repair attempt and go with the far easier jumper fix described above.

Update:  When I did this repair, extruder mapping was not in the firmware version available at that time.   Now it is, and that renders this fix obsolete.  However, you may have to replace a driver chip someday, like I just did when I fried another one. The Allegro chips do not tolerate having their outputs grounded. Having learned from the first disaster, this time I used my milling machine to cut the body of the chip away, leaving just the pins.  These were now easily removed one at a time with a small tip soldering iron.  Mouser's catalog listed the Texas Instruments DRV 8811 as a direct replacement, with a disclaimer, of course.  I looked at the pin arrangement, voltages, and everything looked good, so I bought some.   I soldered one in by first putting a small blob of solder on the circuit board heat sink pad and then heating the board from the back.  The chip nicely settled into place, and then I soldered the pins.  Sure enough, the chip ran the extruder drive nicely.  Interestingly, the motor is much quieter, for the TI chip runs at a lower PWM frequency. The motor also runs slightly cooler, but has the same torque as before.  However, when I tried printing, something was wrong.  Soon I realized that the stepper was running too fast.  Twice as fast as it should.  Then I remembered Mouser's disclaimer and went back over the data sheets for both the Allegro and TI chips.  The Allegro offers full step, 1/2 step, 1/4 step and 1/16 step.  The TI has full step, 1/2 step, 1/4 step and 1/8 step.  The BFB board was set for 1/16, and the TI chip was running at 1/8, or twice as fast.

Rats.  There was no way the fragile pads on the circuit board were going to survive another chip change, and the TI chip is otherwise a very good chip. I decided that instead of removing the chip, I would slow it down by slowing down the signal controlling the chip.  The speed of the stepper is determined by the frequency of the incoming pulses.  So, I decided to cut the frequency in half by making a divide by 2 circuit from a flip-flop. I mounted this chip on a separate board, and then cut the control line to the extruder on the back of the board. 

Wires from left to right:  Ground, Clock input to flip flop, "Q" output from flip flop, and +3.3 volts.
This works perfectly.  My two extruders now track perfectly in speed, with the only noticeable difference is that the motor being driven by the TI chip is quieter.

My divide by 2 trick does not turn the TI chip into a 1/16 microstepper.  Rather, it is more accurately a 1/8 microstepper running at half speed.  This is not a problem on an extruder drive which is running anywhere from 24 to 90 RPM.

Installing a Tailstock Cam Lock on a 7x Mini Lathe

I purchased a Mini Lathe Tailstock Cam Lock from Little Machine Shop.  It is a well made kit.  Actually, I think it is superior to the factory installed locks I've seen on some Mini Lathes.


This is normally an easy to install kit.  However, my tailstock casting was quite uneven where the kit would be resting.  I decided to machine the bottom of the kit to match the angle of my casting.

I had to cut a compound angle and file a radius on the bottom of my kit to get it aligned correctly. Once I had a good fit, I drilled the hole for the shaft.

 

Even though cutting the angle made my kit shorter, I nearly came out of the top of the flat area on the rear of the tailstock.

 Like everything else on this casting, this flat area was not flat, so I milled it flat.   Then I added a flat washer and a wave spring washer to eliminate the sloppy feel of the handle when in the unlocked position.  It works fine without it, especially of you install a spring like the one included in LMS's Tweaks.&.Enhancements kit.  But I like to make everything feel as precise as possible.

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A front view.  I am very pleased with this kit.  It operates smoothly and locks solidly. A vast improvement over the original lock nut.

Upgrading a Sears air compressor with a Harbor Freight pump.

I installed a Harbor Freight 3 HP air compressor unit on my Sears Craftsman compressor, which was a piece of junk that twice snapped it's connecting rods.  While the HF unit is well made, with a sturdy 2 cylinder cast iron block , it was driven by a flimsy cast aluminum pulley with very thin spokes that acted as fan blades.  It ran maybe a hundred hours before one of the spokes broke.  It was clear that it was under a lot of stress from improper annealing after casting, and it broke under tension, leaving a gap of about .003".  I tried to weld it, but could not stress relieve it adequately.  It ran a few hours before snapping the remaining spokes.

I.wasn't about to buy a replacement from Harbor Freight, because it would likely fail also.  The spokes are simply too thin to withstand the stresses put on it by the air compressor.  Maybe I could find a similar pulley from someone else, but it would have to match the tapered shaft of the HF compressor.  I decided to turn it into a machine shop project and rebuild it with stronger spokes made from 1.5" x 0.5" aluminum bar stock.

I began by cutting off all of the spokes and turning the hub smooth in my lathe.  I then set up my boring bar in the mill to match the diameter of the hub.  Then I clamped the new spokes in the vise at a 45 degree angle and used the boring bar to make the end of the spokes match the surface of the hub.

Then I drilled and tapped two 1/4-28 holes one inch apart in the spokes,and corresponding holes in the hub.

I.bolted the spokes in place to check the fit and measure the outside diameter of the spokes.

Next I  trimmed the spokes to length, and cut a 5/16" rounded slot to match a corresponding raised area on the pulley ring.  This locking feature carries the load, allowing me to use relatively small 8-32 screws to attach the ring to the spokes. The screws are underneath the belt, and go straight into the spokes.  Each screw is ground down for clearance and held in place by applying J-B Weld to the threads.

Two spokes installed.  Each spoke has to be filed slightly to compensate for irregularities in the ring casting.  After all spokes are fitted, I plan to bolt everything together while applying J-B Weld to the joints and screws. This should enhance the rigidity and  help it better withstand the considerable vibration that the compressor, which has  no flywheel to smooth it, generates. Hopefully the additional weight of the new pulley will help dampen the vibrations. By vibration I'm referring to rotational vibration, for the torque load briefly drops to near zero every 180 degrees. This causes the belt to shake a bit. Below, the partially completed pulley is receiving a Cat Scan to check for defects.

The completed pulley, balanced and painted:

I installed the Harbor Freight compressor on a Sears Craftsman "6 HP" compressor that was quite honestly, a piece of junk.  It had a cheap compressor unit attached directly to a 2 HP motor turning at 3450 RPM.  It was very loud, and the cheap cast aluminum connecting rods snapped after about a year  of operation.  I bought a rebuild kit that I should have returned as soon as I saw that the connecting rods were as cheap as before.  Even though the kit included new cylinders and I essentially had a new compressor when the parts were installed, the connecting rods snapped after only 20 hours of use.  At this point I was pretty disgusted with this compressor which carries the once-proud Craftsman name.
  In.addition to the cheap design, it was falsely rated as a 6 HP compressor, but had a 2 HP motor.  Anticipating the questions over the discrepancy, Sears ordered motors from A.O. Smith with the HP rating left blank!  See picture below:

The amperage listed is correct for a 2HP motor.  Maybe 2.5HP, but nowhere near 6.  I cut off part of the compressor casting from the end bell of the motor, and replaced the crankshaft with a pulley. I left the second cylinder holder intact and used it as a mount for the belt guard.

I cut the line to the old compressor and used flare fittings to connect a piece of copper tubing to the new compressor. 
Do not remove the T fitting in the tank, for it contains a compression release valve. The pump would not be able to start against the pressure in the tank if this valve is removed.  The small line goes to the release valve on the pressure control switch.
The HF outlet fitting was of a size I didn't have, so I re-tapped it as a pipe fitting and screwed on a half inch flare fitting.


Even though the Harbor Freight pulley failed, overall the HF compressor is far superior to the Craftsman one. It's oil bath design and cast iron cylinder block should last a long time.  Running at only 1000 RPM, it is far quieter than the Craftsman's 3450.  One of the reasons I now find myself at Harbor Freight shopping for tools more often than at Sears.

Update:  My repaired pulley has now been in use for over two years, and is holding up well, with no signs of fatigue or loosening.  However, the compressor blew both the head gasket and exhaust manifold gasket.  I made a new head gasket out of cork, and it is working well.  If you have one of these compressors, be sure to check the head bolt torque periodically.   The exhaust manifold has one very thin area in the gasket that blows easily. I milled a rectangular groove on the manifold mating surface and put a copper wire ring in the groove.  This, plus a new gasket solved the problem.

 Meanwhile, Harbor Freight has replaced this model compressor with a new and better made model.  The pulley is now cast iron. But the price went up from the $99 I paid on sale to $149 sale price for the new one.

RapMan 3D Printer Modifications

I purchased a RapMan 3D printer from Bits From Bytes because it is, as BFB claims, a very cost effective 3D printer.  However, once I had it, I couldn't resist redesigning it. I began by replacing the plastic corner pieces with heavy duty aluminum ones, and didn't stop until I redesigned nearly everything.

The end result bears only a passing resemblance to the original RapMan.

I started by replacing the corner pieces, which consisted of 3 or 4 pieces of polycarbonate and about a dozen M2 screws, with single solid blocks of  1 1/2" x 1 1/2" aluminum.  Then I remembered I had several feet of aluminum C channel, 1/2" outside and 1/4"  inside.  This I decided would make great channels to hold side panels to replace the original X braces.  The result was this foundation for my new machine:

Wherever possible, I tightened the tolerances in my replacement parts.  The bearings now rest in bored holes which hold them in rigid alignment, while the round stock from the original RapMan is tightly pressed into holes in the corner blocks.  The C channel is also pressed into slots, and locked with screws.  The entire foundation is very stiff and square. A 1/4" aluminum plate serves as the motor mount.

Here is a side by side comparison of my aluminum corner assembly vs. the plastic RapMan corner.

The aluminum assembly is comprised of two pieces held together by a milled slot and a single 1/4-28 bolt, while the original consists of 5 pieces of polycarbonate, 12 screws with nuts, and a short piece of 8mm rod.

What must be remembered here is that my design is not economical to manufacture and sell, and that the RapMan design does work OK.  The point I'd like to make is that it is possible to take the affordable RapMan and turn it into the equivalent of a much more expensive machine.

Below: I replaced the idler pulleys made from a ball bearing-and-washer sandwich with a lathe turned pulley riding on an 8mm bearing.

The jack-screws are made from common hardware store threaded rods and nuts, with laser cut polycarbonate toothed pulleys sandwiched between washers. Everything is a sloppy fit, but I'm not complaining, for using common hardware enabled BFB to keep the cost low.  I found the holes in the toothed pulleys to be about .005" over-sized, or about the thickness of a Nine Lives steel cat food can.

This solved two problems: The sloppy fit, and the fact that friction by squeezing the pulley between two nuts was the only thing that locked the pulley to the shaft, for there is no key.  The cat food can made the pulley a tight fit on the shaft, enough to turn the shaft even if the nuts are loose.  At right is a picture of the jack-screw and pulleys, along with a strip from the can.

The original assembly is the one the left.  It has 2 small washers for spacing, then two large washers to form the sides of the pulley.  While the symmetrical appearance looks good, the resulting pulley is too wide.  I settled on the right hand version, which is asymmetrical in appearance, but the correct width for the belt.

I felt the motor pulley deserved special consideration, and made a pair of lathe turned discs to replace the washers. The assembly is held together with the original screws, and clamps to the shaft as before. 

The RapMan merely pinches the ends of the Y axis belts, which can shift over time.  I made clamps with slots that match the teeth on the belts for a more positive grip.

Then I decided that the nut assemblies which support the table were too flimsy for my requirements, so they were the next polycarbonate pieces to go.  Here is a picture of my aluminum replacements.

I retained the basic design, right down to the hex shaped hole for the nut. However, I changed from the two double guides like the original shown here, to four single guides, one at each corner. The double guides help prevent binding on the more flexible original design, but my rigid aluminum design does not require them. The slots that form the inverted "V" in the bottom of my guide are clearance slots for aluminum "C" channel which now frames the table, stiffening it considerably.

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The_completed table and "Z" axis assembly is sturdy and powerful.  Here it is lifting 4 gallons of antifreeze, which is nearly 30 pounds.  Mechanically, it could handle more, but the stepper motor stalled at higher weights.

The completed frame is sturdy, and initially seemed rigid enough.  However, when I tried printing at higher speeds, the frame shook a bit.  Also, I had a misalignment of about .005 inches in the frame from one side to the other.  Adding diagonal braces solved both problems.

Also visible in the first picture is another improvement, a case for the electronics.  As another cost-shaving idea, the electronics come with only a top panel, while the sides and back are exposed. This does not present a hazard to the operator, but it does make the electronics vulnerable.  In addition, there is no strain relief for the wires.

Here is a rear view.  The wires go through the holes in the board, and that is it. 

I fabricated a case from the metal case of an old VCR.  VCR's are a great source of a variety of parts. I wished to keep the original RapMan electronics assembly as original as possible, but I did make 2 modifications. First I countersunk the screws in the top cover for a nicer appearance, and then I milled a relief around the perimeter so the cover would fit flush with the surface of my case. 

The electronics slide in from the side, and are retained by polycarbonate pieces which lock everything tightly in place.   The cable protectors are twisted into threaded holes in the polycarbonate and will not pull loose.

The picture below shows how the side pieces support the electronics cover and circuit board.

Below, a close up of the assembled controls. The side panels are .10" steel, and the whole thing feels very substantial.  There is plenty of room for air circulation underneath.

The screw terminal "D" connector will be replaced with a conventional one, further improving the appearance.

 With all of my experimenting, I have now fried two stepper motor driver chips. I repaired the board, but some of my repairs are a little unusual. For more on that, click here



More to come!  After going this far, I've decided to redesign the carriage the print head rides on.  There will be very little polycarbonate left, and the the machine will gain a few more pounds in weight.  It will likely tip the scales at around 60 pounds when complete.

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The print head development took a while, for I developed an entirely new design.

The Rap Man printer head works OK, but is excessively large in size, and is complicated, with the body made from 10 pieces of laser cut Plexiglas. The head is over-sized because the motor , feed screw, gears and bearings are mounted at an angle while the raw material being fed to the printer is vertical.  BFB did this to move the drive gear and bearings out of the way of the path of the raw material.

In  the more expensive BFB3000, they made the head smaller by mounting the drive gear vertical and bending the raw material.  This also works OK, but is a nuisance to reload, as one has to snake the material through the curved path.

My idea was to have the best of both worlds by positioning everything vertical.  This would make the head small, and reloading simple.  But, of course that meant the bearings would be in the way.  The solution here was to move the bearings out of the way.  But how to do it?  Instead of having the drive screw go through the center of the bearings, as is normally done, I put the drive screw on the outside of the bearings. Now, instead of the center of the bearing rotating while the outside is stationary, the outside rotates, and the inside is stationary.

This required two bearings at each end.  The feed screw now rides in the valley between the two bearings, leaving the top of the feed screw unobstructed, creating a straight path for the raw material.  

Below is a picture of my feed mechanism. 

Very compact, and a much more attractive design than the original RapMan head.

Now that the material feed design was complete, it was time to develop a mounting system. Primary requirements were ease of removal, and a versatile design which would permit me to experiment with a variety of print head designs, including the ability to swap the 3D print heads with a mechanical or laser engraver head, or perhaps even a pottery extruding head.

 

I settled on a dovetail style mount, which eliminated the need for threaded holes in the carriage, and bolts to attach the heads.  A single screw on each print head expands the dovetails, clamping them in position quickly and securely.

The carriage is machined from a solid block of 1 inch thick aluminum.   This allowed me to make a very low profile carriage whose top surface is just 2 mm above the linear bearings.  This helps to reduce the overall height of the machine. Combined with the low profile print heads, my machine is about 4 inches lower than the RapMan.  My carriage is also smaller in both the X and Y dimensions, resulting in more x and Y travel than the RapMan carriage.

 

One design problem I struggled with was how to get power to the extruders in a way that looked tidy and also gave me the versatility to experiment with different styles of heads.  I rummaged through my collection of old Apple hardware and found a 50 conductor ribbon cable from a hard drive.  It was an ideal length, and I was able to cut the connectors off the hard drive and motherboard.

I cut one of the connectors up into several small ones, and made individual plugs for each motor and extruder.  This way I can easily remove them separately, or swap their positions for troubleshooting.  

Unlike  the RapMan, which required removing several screws and wires, I can remove each head in under a minute by loosening only a single screw.

Since ribbon cable has light gauge wires intended for carrying signals, and not large amounts of power, I used 3 wires in parallel for each extruder and stepper motor.  This arrangement works well, giving me the flexibility of a ribbon cable, and the current carrying capacity of a thicker, and stiffer, wire. Out of the 50 conductors available in the ribbon, I used 46.

Finally, I wanted a convenient means to hold several spools of material without taking up valuable table space.  I mounted the spools on top of the machine, where they are readily accessible.  A picture of the completed machine is below. The whole thing fits nicely on a small 2 foot x 2 foot  table.

  I ran the "Duck" test program provided by BFB, and made some very nice ducks in a variety of colors.  The surface finish is quite good, and I'm impressed with what can be done without using support structures.  These ducks are ABS, but I've also made them in PLA.

 I modified the duck file to make two color ducks as a test of the second extruder head:

After making a whole flock of ducks and other things, I realized that there is still room for improvement.  One thing I discovered is that PLA stays soft for a longer time than ABS.  This made it difficult to make small parts because the material was too rubbery and unstable when the extruder returned for a second pass.   I solved this by adding a second fan on the rear of my carriage.  Below are pictures of the brackets, and how they mount on the carriage.

 This solved one problem , but created another.  The RapMan extruders always struggled to reach maximum temperature, and now with the increased airflow, and the addition of a heat sink on the inlet side, they really struggled. It didn't help that BFB did not insulate them well, or used 3 stainless steel standoffs to mount them.  I turned the center portion of the standoffs down in my lathe to about 2.6mm to reduce the heat transfer.  As the cross sectional area of the standoff is now less than half the original, the heat transfer should also be reduced.

 I added additional high temperature RTV to the exposed back of the extruder, and covered the slimmed down standoffs. It did not make a huge improvement, but it was enough to allow the heads to maintain 260 degrees.

I ran these extruders for many hours, and they generally worked well, but would occasionally lose their grip on the filament.  Increasing the pressure on the filament against the screw helped, but then the motor would occasionally stall.  I realized that the blunt machine screw style threads took considerable pressure to bite into the filament, so I designed a much sharper tooth profile.  Below is a drawing representing the original profile and my design.










I made these screws from water hardening drill rod, which I hardened after machining.    The new screws are working well so far, and require much less pressure to maintain a grip on the filament.  It remains to be seen whether the hardened drill rod maintains a sharp edge.






Another improvement was made to the material feed arrangement.  Feeding from overhead quickly proved to be less than ideal, so I added a pair of turning pulleys to bring the material in from behind.  The top pulley removes quickly without tools.  With room for 6 materials overhead, material changing is far easier than most other machines I've seen.

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With the machine now running well, I decided to tackle the issue of ABS shrinkage and build a heated bed.  Since my existing bed was made from 1/8 inch aluminum, I was off to a good start.  I experimented with a variety of heaters and decided that 300 watts was about right.  The heating elements were removed from food warming trays which had a nice length of Nichrome wire insulated with fiberglass.  I would not recommend going beyond 300 watts, for if your temperature controller sticks on, the bed would get extremely hot.

I had a old British made CAL 5000 temperature controller in my junk box, so I built an enclosure and used that.  It uses a type J thermocouple.  The heated bed was a huge improvement when printing in ABS.  It also helps when using PLA as a raft material, for it keeps the PLA slightly soft and sticky.  
     Next problem.  I was not satisfied with Kapton as a bed surface, and I heard of someone using a stone surface, so I decided to give it a try. I tried a marble tile and that has worked well so far.  I'm using the rough back side of the tile as my surface. As long as the tile is warm, the PLA raft sticks well. When it cools, the PLA pops off by itself.



Finally, here is my copy of Emmett's Heart Gears, a popular download from the Thingiverse site: